Botkin Environmental Science Earth as Living Planet 8th txtbk

336 CHAPTER 16 Alternative Energy and the Environment
on the north coast of France. Constructed in the 1960s, it
is the first and largest modern tidal power plant and has
remained in operation since. The plant at capacity produces
about 240,000 kW from 24 power units spread out across
the dam. At the Rance power plant, most electricity is produced
from the ebb tide, which is easier to control.
Tidal power, too, has environmental impacts. The
dam changes the hydrology of a bay or an estuary, which
can adversely affect the vegetation and wildlife. The dam
restricts upstream and downstream passage of fish, and
the periodic rapid filling and emptying of the bay as the
dam opens and closes with the tides rapidly changes habitats
for birds and other organisms.
16.6 Wind Power
Wind power, like solar power, has evolved over a long
time. From early Chinese and Persian civilizations to the
present, wind has propelled ships and has driven windmills
to grind grain and pump water. In the past, thousands
of windmills in the western United States were used
to pump water for ranches. More recently, wind has been
used to generate electricity. The trouble is, wind tends to
be highly variable in time, place, and intensity. 18
Basics of Wind Power
Winds are produced when differential heating of Earth’s
surface creates air masses with differing heat contents
and densities. The potential for energy from the wind is
large, and thus wind “prospecting” has become an important
endeavor. On a national scale, regions with the
greatest potential are the Pacific Northwest coastal area,
the coastal region of the northeastern United States, and
a belt within the Great Plains extending from northern
Texas through the Rocky Mountain states and the Dakotas
(Figure 16.13). Other windy sites include mountain
areas in North Carolina and the northern Coachella Valley
in Southern California. A site with average wind velocity
of about 18 kilometers per hour (11 mph) or greater is
considered a good prospect for wind energy development,
although starting speeds for modern wind turbines can be
considerably lower. 19
In any location, the wind’s direction, velocity, and duration
may be quite variable, depending on local topography
and temperature differences in the atmosphere. For example,
wind velocity often increases over hilltops and when wind
is funneled through a mountain pass (Figure 16.14). The
increase in wind velocity over a mountain is due to a vertical
convergence of wind, whereas in a pass the increase is
partly due to a horizontal convergence. Because the shape of
a mountain or a pass is often related to the local or regional
geology, prospecting for wind energy is a geologic as well as
a geographic and meteorological task. The wind energy potential
of a region or site is determined by instruments that
measure and monitor over time the strength, direction, and
duration of the wind.
Significant improvements in the size of windmills
and the amount of power they produce occurred from
the late 1800s to the present, when many European
countries and the United States became interested in
Wind speed m/s
> 10.0
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
< 4.0
United States - Annual Average Wind Speed at 80 m
FIGURE 16.13 Wind energy potential in the United States.